As the world's conventional oil reserves are being consumed, Heavy Oil and Tar Sands (HOTS) reservoirs are becoming increasingly important as a source of petroleum fluids. However, recovery of oil from HOTS reservoirs is generally difficult due to the high viscosity and poor mobility of oil, difficult production and high fluid property heterogeneity in reservoirs. Of the world's petroleum reserves (6 trillion barrels), most exists as heavy oil or oil sand (tar sand) bitumen. Currently, only an average of 17% of this oil can be recovered.
In high porosity and permeability reservoirs, development strategy and production depends on fluid mobility, of which oil viscosity is a major controlling factor. Thus, assessing viscosity of oil throughout HOTS reservoirs is one key to the design and operation of recovery strategies for the production of these high viscosity fluids. This is mostly done by measuring the viscosity of a gas-free bitumen or oil to obtain a dead oil viscosity which can be converted to an in situ live oil viscosity using estimates of bitumen gas content in situ. Ideally, however, the viscosity should be measured directly on a live oil sample that contains the in situ gas content of the reservoir sample, which is mostly not possible due to limitations of the methods by which oil or bitumen is extracted from samples and by which viscosity measured. The large scale fluid property heterogeneity seen across HOTS reservoirs requires detailed fluid property measurement on unaltered oil samples (derived from fresh, “pristine” reservoir samples) and free of mineral fines and water to map viscosity variations in reservoir.
As part of the production appraisal process, it is routine to measure the physical properties (e.g. viscosity, API gravity) and chemical properties such as sulphur content, of the bitumen and heavy oil contained in the reservoirs at closely spaced intervals (every 5 to 10 m or even closer vertically spaced intervals) over the thickness of the reservoir. These analyses must be performed on a water and sediment-free, chemically unaltered representative bitumen or heavy oil sample, which is in the form of liquid petroleum. While analyses can be performed on stored frozen reservoir core samples in the laboratory, ideally these separations of heavy oil or bitumen from reservoir core samples and subsequent viscosity or other measurements should be performed rapidly at the well-site, such that the acquired information from the analyses can be used to assist drilling decisions, particularly with regards to decisions regarding sidetracked wells. For instance, measured oil viscosity may provide useful information to assist in determining if that section of the reservoir is suitable for production.
Presently, techniques of solvent extraction to recover a bitumen sample for viscosity measurement alter the physical properties of heavy oil and tar sand bitumen. Usually, solvents cannot be completely removed without losing some of the low molecular weight petroleum components within the bitumen. Thus, viscosity measurements on solvent extracted bitumen are considered unreliable with existing procedures.
It is possible to mechanically extract heavy oil and tar sand by high speed centrifugation. However, this method proceeds slowly due to the high viscosity of the petroleum in many samples and cannot be performed easily or quickly enough at the well-site. Moreover, even if time is available in a laboratory the expense of engineering-rated centrifuges raises costs dramatically. Furthermore, centrifugation can alter the true composition of the sample by loss of volatile bitumen components, or produce samples of bitumen admixed together with water and fines.
In addition, large quantities of reservoir rock (>0.5 kg or equivalent to a minimum of 30 to 50 cm lengths of slabbed 4″ core) are usually required to acquire adequate samples for fluid property determinations, which is both destructive and difficult to handle.
Other methods of recovering very viscous bitumen also include displacement of petroleum with very viscous fluids such as silicone. These techniques are both time-consuming and expensive or ineffective as often insufficient liquid samples are obtained for subsequent physical property testing.
At present, these slow mechanical laboratory petroleum recovery processes are a bottleneck in most bitumen or very heavy oil production planning operations. Thus, there has been a need to provide a rapid and effective recovery method that provides unaltered water and unaltered, sediment free oil or bitumen for viscosity and other analysis, the bitumen or oil retaining its low boiling light end components A review of the prior art indicates that such a system is not known to the inventors.